The side-by-side images show the Air Quality Index (which measures the number of pollutants in the air) at 29 for the U.S. Embassy in Beijing and 174 at a Colorado Department of Public Health and Environment air quality reporting site in downtown Denver.

9NEWS’ Verify team decided to check whether that was true.

'BROWN CLOUD'

The haze over Denver has affectionately been referred to as the "brown cloud." Professor Alex Huffman teaches chemistry at the University of Denver. He said the "brown cloud" has been around for decades.

“In the back of old textbooks, you can look up Denver Brown Cloud,” Huffman said. “It was a significant enough problem here in Denver that we made textbooks unfortunately.”

The brown cloud is created after what’s called a temperature inversion. That’s when a layer of warm air traps a layer of cold air beneath it close to the ground. The brown cloud is the stuff that gets sandwiched between.

Haze in Denver (1/8/17)

“You have a layer of warm air that gets stuck on top of that layer of cold air beneath [and it] condenses that layer of pollution and you see it a little more,” Huffman explained.

Huffman said the brown layer of pollution is made up of fine particulate matter like dust and nitrogen dioxide gas.

WHAT WE FOUND

Now, back to the tweet. It's actually correct. The air quality in Denver Monday was worse than Beijing.

But that’s not the whole story.

1. The number for Denver isn’t quite right.

The people at CDPHE who track air quality readings for Denver never saw a reading as high as the AirVisual app Dempsey used.

“It is possible that their AQI reporting of 174 at the CAMP monitor used a different method of measurement than the one we use, by regulation, to report and forecast air quality,” CDPHE spokesperson Alex Niebergall said. “Our highest AQI reported at CAMP in the last 24 hours was 111.”

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9NEWS emailed AirVisual to ask how it gets it numbers, but we didn’t hear back.

Still, 111 is still bigger than 29 – the number reported for Beijing. And that number looks to be correct.

This is important because the number of pollutants in the air change depending on the time of day.

“Ozone levels often peak in the afternoon to early evening,” according to the Environmental Protection Agency’s website on air quality. “Carbon monoxide may be a problem during morning or evening rush hours. And particle pollution can be high any time of day, and is often elevated near busy roadways, especially during morning or evening rush hours.”

Haze in Denver (1/8/17)

The data point from Denver was taken at 5 p.m. Monday – during rush hour -- while the data point in Beijing was at 9 a.m. Tuesday.

The air in Beijing got worse as its Tuesday wore one hitting a high of 89, according to China’s AQI website.

Still, if you take the worst numbers from each city from the same 24-hour time period, you see that Denver’s air was worse than Beijing’s.

3. Denver experienced an inversion Monday.

An inversion is a weather event that acts like the lid of a jar when it comes to pollution. It traps pollutants near the surface of the earth and keeps them from mixing out into the atmosphere.

“Typically inversions will occur in the early morning and lessen throughout the day … ,” Niebergall said. “Yesterday's inversion layer was atypical because it actually got stronger throughout the day.”

Combine that with rush hour traffic and you get an air quality reading reminiscent of Denver in the 1980s.

“Exceptional weather events can cause brief spikes in pollution levels, but this is not representative of air quality in Colorado as a whole,” Neibergall said.

4. On average, Denver’s air is a lot cleaner.

Denver had a bad day Monday when it comes to air quality. But Beijing, on average, has a lot more bad days. And its bad readings are a lot worse.

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Denver’s average AQI is within the good air range while Beijing’s average still falls in the unhealthy category, Neibergall said.

An AQI above “300 represents air quality so hazardous that everyone may experience serious effect,” according to the EPA’s air quality website.

Thankfully, those kinds of numbers are “extremely rare” in the U.S. The air generally only gets that bad here during a forest fire.

But in China people still experience off the chart days, meaning the AQI score tops 500.